This invention relates to optical communications and more specifically to amplification of optical signals.
Optical communications are an important field. Typically optical signals are carried on optical fibers from a transmitter to a receiver. Such systems typically include optical amplifiers that amplify the signals in the optical (not electrical) regime. Such systems are used, for instance, in telecommunications and cable television. The growing demand for more bandwidth (ability to carry more information on a particular fiber) has expanded the use of what is called wavelength division multiplexing. This means one fiber carries simultaneously optical signals at several different wavelengths (frequencies). Presently available optical equipment including the transmitters, receivers and optical amplifiers typically operates in the spectrum of 1530-1625 nanometers. The most commonly used portion of this spectrum is the C band which is 1530-1565 nanometers. Immediately above the C band is the L band which is about 1565-1625 nanometers. The S band is below the C band at about 1460-1530 nanometers. Most presently available optical equipment operates in the C band. One reason for this is that one of the key components, which are the optical amplifiers, are typically erbium-doped fiber amplifiers (EDFA). An EDFA includes a length of optical fiber doped with the element erbium and which is pumped with a pump laser. These erbium-doped fiber amplifiers typically amplify wavelengths in the C band. Erbium-doped fiber amplifiers also can amplify wavelengths in the L band. However, for physical reasons they are not capable of amplifying wavelengths in the S band. This is a significant drawback since other types of optical amplifiers have major shortcomings and it is very desirable to use the S band in wavelength division multiplexed optical communication systems. However, this shortcoming of EDFA""s generally prevents that. Note that a typical optical communication system requires amplification approximately every 80 kilometers of optical fiber due to signal losses in the fiber. It is of course possible to amplify signals in the electrical domain using a regenerator which converts the optical signal to an electrical signal, amplifies the electrical signal, and then reconverts it to an optical signal. However, this process involves its own losses and the regenerators are complex and expensive, so it is desirable to avoid their use if possible.
Hence, in the prior art there still remains the problem of the inability to effectively use the S band for optical communication without converting the signal into the electrical regime and reconverting it back to the optical regime.
In accordance with this invention, an optical amplifier is provided which allows amplification in the optical regime of S band optical signals. In one embodiment this is done by taking the S band signal, converting it using an optical wavelength converter to a C band wavelength signal, amplifying the C band signal using an EDFA, then reconverting the amplified C band signal back into an S band signal. This uses a first wavelength converter which converts the S band signal to the C band and a second wavelength converter which converts the amplified C band signal back to the S band. Typically, the amplifier in addition to operating in the S band also includes paths for C band and L band signals which themselves are amplified using their own EDFA, without conversion.
In another embodiment, the C band and S band signals are each converted to the other band in one amplifier stage, the converted signals are propagated along a span of fiber, and then the converted signals are reconverted to their original bands in the next amplifier stage. In each stage only one of the C band or S band signals is amplified. The L band signals similarly are amplified in every other amplifier stage. This requires somewhat shorter spans of fiber between stages than other embodiments but provides better power balance.
The conversion of the S band to the C band and back again is performed completely in the optical regime in these embodiments. Moreover, such a system allows wavelength division multiplexing of S band, C band, and L band signals using coarse demultiplexers and multiplexers to separate the three bands and amplify them each using a separate path including an EDFA. The multiplexing and demultiplexing are performed using conventional optical components.
The wavelength converters used to convert the S band to C band and vice versa are, for instance, optical devices based on non-linear optical materials which shift entire bands of wavelengths to the desired other band. One example of such a device is a converter employing difference frequency generation (DFG) techniques in a PPLN (periodically poled lithium niobate) waveguide.